1. Field of the Invention
The invention relates to power supplies, more particularly to a half-bridge self-exciting switching power supply with an over-voltage cut-off capability.
2. Description of the Related Art
Referring to FIG. 1, a conventional half-bridge self-exciting switching power supply is shown to comprise a rectifier circuit 1, a pulse-width-modulated inverter 2 and a power transformer unit 3. The rectifier circuit 1 includes a bridge rectifier (BD1) and capacitor filters (C1, C2). The pulse-width-modulated inverter 2 includes a half-bridge self-excited circuit 20, a pulse-width-modulated (PWM) controller 21 and a driving unit 22. The half-bridge self-excited circuit 20 is connected to the rectifier circuit 1 and includes a first self-excited switching circuit with a first transistor (Q1), and a second self-excited switching circuit with a second transistor (Q2). The driving unit 22 includes a driving transformer (T2) coupled to the first and second self-excited switching circuits of the half-bridge self-excited circuit 20, and first and second driving transistors (Q3, Q4). The power transformer unit 3 includes an isolation transformer (T1) coupled to the driving unit 22 and connected to the rectifier circuit 1. The isolation transformer (T1) provides two output voltages which are rectified and filtered by rectifier and filter circuits (not shown) to obtain two dc power supply outputs, namely +5V and +12V. The PWM controller 21 includes a voltage feedback circuit 210 which receives the dc power supply outputs, and a pulse-width-modulating unit (IC1), such as the 7500 or TL494 integrated circuit by Samsung. The pulse-width-modulating unit (IC1) is connected to the voltage feedback circuit 210 and the driving unit 22.
In operation, a 230V/110V ac line voltage input is processed by the bridge rectifier (BD1) of the rectifier circuit 1 before being filtered by the capacitor filters (C1, C2) in order to generate a higher voltage rectified dc signal, e.g. 300 volts, which is supplied to the half-bridge self-excited circuit 20 of the pulse-width-modulated inverter 2. Upon biasing, the first transistor (Q1) of the first self-excited switching circuit of the half-bridge self-excited circuit 20 conducts, thereby generating a current (I1) in the first self-excited switching circuit for energizing the driving transformer (T2) of the driving unit 22. Upon saturation of the first transistor (Q1), the first self-excited switching circuit is cut-off, and the second transistor (Q2) of the second self-excited switching circuit of the half-bridge self-excited circuit 20 is biased into conduction, thereby generating a current (I2) in the second self-excited switching circuit for continued energizing of the driving transformer (T2). The second self-excited switching circuit is cut-off when the second transistor (Q2) becomes saturated. The alternating conduction of the first and second self-excited switching circuits is referred to as a half-bridge self-exciting operation. Energizing of the driving transformer (T2) by the currents (I1, I2) of the first and second self-excited switching circuits enables the power transformer unit 3 to provide the +5V and +12V dc power supply outputs.
The voltage feedback circuit 210 of the PWM controller 21 provides the dc power supply outputs to the pulse-width-modulating unit (IC1). Upon comparing the dc power supply outputs with reference voltage values, the pulse-width-modulating unit (IC1) generates pulse drive signals for the first and second driving transistors (Q3, Q4) of the driving unit 22, thereby controlling the on-off ratios of the first and second self-excited switching circuits of the half-bridge self-excited circuit 20.
It can be understood from the foregoing that the conventional half-bridge self-exciting switching power supply relies primarily on the PWM controller 21 and the driving unit 22 to prevent over-voltage of the power supply outputs. However, if the PWM controller 21 or the driving unit 22 becomes defective, the self-exciting operation of the first and second self-excited switching circuits will go on uninterrupted. This is possible when the pulse-width-modulating unit (IC1) is unable to compare the dc power supply outputs with the reference voltage values, or when one of the first and second driving transistors (Q3, Q4) or the driving transformer (T2) is damaged. Uninterrupted self-exciting operation of the first and second self-excited switching circuits will result in the dc power supply outputs exceeding the required +5V and +12V values, thereby damaging the load (not shown) that is connected to the switching power supply. This is best illustrated in FIG. 2. After the power supply output peaks at (A), the power supply output then decreases due to internal damage of the switching power supply, as shown at (B) in FIG. 2.